This invention relates to an improved process for reacting alkynes with a hydroxyl compound in the presence of a catalyst and to an improved process for separating alkynes from fluid mixtures.
Hydrocarbon conversion processes yield crude C.sub.4 fractions which contain valuable components such as butadiene. A typical such fraction is that given in Table I.
TABLE I ______________________________________ Volume Boiling Component Percentage Point .degree.C. ______________________________________ 1,3-butadiene 39.1 -4.4 isobutylene 27.7 -6.9 1-butene 17.2 -6.3 trans-2-butene 6.0 +0.9 cis-2-butene 4.5 +0.9 n-butane 4.1 -0.5 C.sub.3 hydrocarbons 0.9 -- C.sub.4 acetylene 0.2 +5.1 C.sub.5 hydrocarbons 0.1 -- 1,2-butadiene &lt;0.1 +10.9 ______________________________________
1,3-Butadiene is a commercial valuable chemical which ranked 31st in order of high volume chemicals produced in the United States in 1975. 1,3-Butadiene is of great importance in synthetic rubber manufacture generally requiring a minimum purity of at least 99.0 weight percent. Generally, the maximum allowable amount of acetylenes as trace impurities is 500 ppm. Higher amounts cause undesirable polymerization of the acetylene contributing to equipment fouling and foaming problems.
A common method of purifying 1,3-butadiene from a C.sub.4 hydrocarbon stream is a two-stage extractive distillation process. In this process, butanes, butenes and generally compounds less polar than butadiene are removed by extractive distillation with an appropriate solvent. Next, a second extractive distillation is utilized to remove alkynes and those compounds more polar than 1,3-butadiene. This two-step process is rather energy intensive which results in additional capital costs for heat recovery in order to minimize operational costs. Also alkynes such as vinylacetylene and diacetylene are unstable compounds which can be highly dangerous when concentrated. In order to lessen the above-mentioned problem, some loss of butadiene must be tolerated in order to remove the alkynes at low concentrations. Conventional purification processes are outlined in greater detail in The Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 4, 3rd Ed., p. 326 (1978).
In 1966 a process was proposed in U.S. Pat. No. 3,273,314 (Quinn) involving the removal of alkynes by absorption with a silver carboxylate ion-exchange resin. This process is primarily directed toward the removal of alkynes from gaseous streams such as ethylene or helium. The process has the disadvantages of requiring frequent regeneration due to exhaustion of the alkyne capacity of the resin. Regeneration requires treatment with nitric acid and necessitates removal of the resin being regenerated from operation. This regeneration is both time consuming and costly.
In 1969, U.S. Pat. No. 3,458,591 (Bebb et al.) proposed a method for removing alpha-acetylenes from a hydrocarbon mixture containing 1,2-butadiene by a separation process which utilized an aqueous solution of sulfuric acid containing mercuric ions. Following contact with this solution, a phase separation allows the treated hydrocarbon to be separated from the acid layer. Bebb et al. proposed the following reaction: ##STR1## Apparently the above reaction forms products which are more polar and therefore segregate in a phase separate from that of the desired product. The Bebb method requires contact times of at least an hour and preferably at least two hours with very efficient agitation to effectively remove alkynes. If less efficient agitation is employed, much longer contact times are required. Other disadvantages are the extra environmental and safety precautions required by the use of mercury compounds which are extremely toxic.
During 1977 and 1978 four U.S. patents assigned to Snam Progetti, S.p.a., issued relating to the removal of alkynes from hydrocarbon mixtures having U.S. Pat. Nos. 4,020,114 (Rescalli I); 4,031,157 (Rescalli II); 4,066,713 (Rescalli III) and 4,112,009 (Rescalli IV).
Rescalli I proposed a process for the separation of butadiene from a C.sub.4 hydrocarbon stream involving a sequential etherification of isobutylene and the acetylenic compounds followed by a distillation step. Rescalli II is directed to a method for removing acetylenic compounds from hydrocarbon mixtures involving etherification with an alcohol or glycol with removal of the formed ethers by distillation. Rescalli III involves removal of acetylenic compounds contained in inorganic or organic hydrocarbon streams characterized in that an organic acid is added to the acetylenic compounds. Rescalli IV is directed to a method for removing acetylenic compounds from hydrocarbons by reacting the acetylenic compounds with compounds of the formula R--OH wherein R is acetyl and thereafter removing the products. Common to all Rescalli patents I, II, III and IV is the feature of contacting the reactants of each process with an acid ion-exchange resin containing mercuric ions. All four patents prefer that such ion-exchange resin have polystyrene or polyphenolic matrix and three prefer also a divinylbenzene matrix. All four also prefer that the matrix have as substituents sulfonic groups (--SO.sub.3 H) and mention that resins having pendant --COOH groups are also useful.
All of these processes have the disadvantage of working with mercury whose toxicity is well-known. Also ion-exchange resins containing mercuric ions are not as selective toward alkyne conversion as is desired with isobutylene also being converted.